In machine communication, a terminal operates in a Power Saving Mode (PSM), an extended Discontinuous Reception (DRX) mode, or another mode for the purpose of saving power, where the extended DRX mode refers to an idle DRX mode with the longest periodicity of 10 seconds. The terminal operating in the power saving mode or the extended DRX mode may not respond to a downlink paging message in a timely manner, thus resulting in some transmission delay in transmission of service data.
The project “Communication with a High Delay” of the 3GPP SA2 working team is primarily focused on how user-plane data are transmitted over the network to the terminal while failing to respond to a downlink paging message in a timely manner. At present, there are a number of solutions proposed in TR23.709 including the solution 2 where generally a serving Gateway (SGW) buffers downlink data, and while the terminal is contacting the network, the SGW transmits the buffered data to the terminal through a relaying base station.
As illustrated in FIG. 1, the solution 2 in TR23.709 is particularly as follows.
S101: The SGW receives downlink data of a Packet Data Gateway (PGW) to be transmitted to a specified User Equipment (UE).
S102: The SGW transmits a Downlink Data Notification (DDN) message to a Mobility Management Entity (MME) or a Serving General Packet Radio System (GPRS) Support Node (SGSN).
S103: The MME or the SGSN detecting the UE in the PSM, the extended DRX mode, or another temporarily unreachable state, the returns to the SGW a DDN Acknowledge (ACK) message carrying a request for buffering downlink data of the UE, and indicates a length of time T for which the SGW buffers the downlink data.
S104: The SGW buffers the downlink data for the UE according to the indication of the MME or the SGSN, and starts a timer S with the length of time T, that is, buffers the downlink data of the UE for the length of time T.
S105: The MME or the SGSN establishes a session with an evolved Node B (eNB) (or a Radio Network Controller (RNC) or a Base Station Controller (BSC)) serving the UE, the UE establishes a session with the eNB (or the RNC or the BSC), after some delay, and the UE initiates a Service Request procedure to the SGW.
S106: The UE initiates a Tracking Area Update (TAU) or a Routing Area Update (RAU) request to the MME or the SGSN, and an evolved Radio Access Bearer (E-RAB) or a Radio Access Bearer (RAB) is established between the UE and the SGW in the TAU or RAU procedure.
S107: The SGW transmits the buffered downlink data of the UE to the UE before the timer S expires, and the SGW deletes the buffered downlink data of the UE when the SGW detects that the timer S expires.
S108: Data are communicated bi-directionally between the UE and the SGW.
Here the MME or the SGSN initiates a paging procedure to the UE, and the UE initiates a service request procedure upon reception of a downlink paging message, and establishes a user-plane bearer with the network during the service request procedure, where the user-plane bearer includes two sections, i.e. one between the UE and the eNB, and the other one between the eNB and the SGW, and data can be transmitted between the UE and the network after the user-plane bearer is established. Since the UE operates in the PSM or the extended DRX mode, the UE which has entered the idle state fails to respond to a downlink paging message in a timely manner.
The MME indicates the SGW to buffer downlink data for the UE, and indicates the length of time T for which the SGW buffers the downlink data, when the MME detects the UE entering the PSM or the extended DRX mode. The SGW buffers the data for the UE according to the indication of the MME, and starts a timer S with the length of time T.
Before the timer S expires, the UE responds to paging by the network, or the UE which has data to be transmitted initiates the service request procedure on its own initiative, or the UE is in the TAU procedure, the SGW transmits the buffered data to the UE. The TAU procedure may be periodical, or may be triggered by the UE moving to a new Tracking Area (TA).
Here if the UE operates in the extended DRX mode, then the value of T will not be less than the periodicity of the extended DRX mode; and if the UE operates in the PSM, then the value of T will not be less than a remaining period of time before a periodical update procedure is initiated next time.
Here if the MME indicates the SGW to buffer downlink data of the UE, then the MME will pass the indication to a new MME in the TAU procedure. In the TAU interaction procedure between the UE and the network, the MME establishes a user-plane bearer for the UE, and also transmits a TAU Accept message.
In the TAU procedure in TS23.401, if the UE needs to transmit user-plane data, then the UE will set an Active Flag in a TAU request message, and if the Active Flag is set in the TAU request message, then a user-plane establishment procedure will be performed, and also the TAU Accept message will be transmitted, as described in TS36.300. Particularly as illustrated in FIG. 2, in the TAU procedure, the MME or the SGSN before the update is enabled, referred to as the old MME or the old SGSN, carries a Downlink Data Forward Request in a context response message transmitted to the new MME which is updated during the TAU procedure, where the request indicates downlink data to be forwarded. The new SGW carries an IP address and a Tunneling Endpoint Identifier (TEID) of a User Plane Part of GPRS Tunneling Protocol (GTP) which are allocated for forwarding of downlink data, in a Session Establish Response message transmitted to the new MME, where the IP address and the TEID are passed to the old SGW which is not updated in a subsequent TAU interaction procedure. The old SGW forwards the buffered downlink data to the new SGW according to the IP address and the TEID. After all the data are transmitted, a data packet carrying an end indication is transmitted to the new SGW, where the end indication indicates that all the downlink data to be forwarded have been transmitted.
In the TAU procedure, it is only indicated that the UE needs to set an Active Flag, and the network establishes a user-plane bearer for the UE during the TAU procedure according to the Active Flag, and the network can perform an operation combining the establishment of the user-plane bearer and the transmission of the TAU Accept message.
In TS23.709, the UE may not contact with the network before the timer S expires, due to an unpredictable network condition, and after the timer S expires, if the SGW has deleted the buffered downlink data of the UE, and the UE has initiated an TAU procedure, and has no data to be transmitted to the network, then if the MME initiates a user-plane establishment procedure between the UE and the eNB, and the SGW has no downlink data to be transmitted, then there may be an unnecessary signaling overhead.